A ２５-year-old male was referred to our hospital for a suspected enlargement of a mediastinal mass on chest radiograph. CT scan and MRI showed a smooth ３３×２７ mm left posterior mediastinal mass at the level of the first and second thoracic vertebrae, suggestive of ganglioneuroma or schwannoma. Although the patient was asymptomatic when referred, due to the possibility of becoming symptomatic, surgery was planned. Video-assisted thoracic surgery was performed. We diagnosed the mass as schwannoma from the second thoracic sympathetic ganglion, and resected the mass partly subcapsular to spare the sympathetic ganglion, but resected the nerve at the peripheral end of the ganglion. Pathology confirmed the mass as Antoni A type schwannoma, and no malignant cells were found. Slight left ptosis and left palmar hypohidrosis was noted after surgery, but improvement was seen １ year after surgery.